1. Field of the Invention
The present invention relates to a charge transfer device used for a solid state imaging apparatus and the like.
2. Description of the Related Art
In the past, a horizontal transfer charge coupled device (HCCD) has been used for transferring a signal charge in a horizontal direction in a CCD-type solid state imaging device. The HCCD is driven by a two-phase clock using a double-layered polysilicon electrode. FIG. 7A is a cross-sectional view schematically showing a conventional HCCD 700 driven by a two-phase clock. FIG. 7B is a plan view thereof. The HCCD 700 includes an n-type semiconductor substrate 701, and first and second polysilicon electrodes 703 and 704 respectively formed on an insulating film 702 provided on the n-type semiconductor substrate 701. In the n-type semiconductor substrate 701, p-regions 705 are formed below the respective second polysilicon electrodes 704 so as to form a potential barrier. Each pair of the first polysilicon electrode 703 and the second polysilicon electrode 704 is electrically connected to a first wiring 708 and a second wiring 709 in an alternate manner through contact holes 707 which are formed in an insulating film (not shown) over the first and second polysilicon electrodes 703 and 704. A signal charge is transferred in the n-type semiconductor substrate 701 by applying two drive pulses (the phase of one pulse being different from that of the other pulse by 180 degrees) to the first wiring 708 and the second wiring 709, respectively.
As is shown in FIG. 7B, the first polysilicon electrode 703 and the second polysilicon electrode 704 have a width L1 and a width L2, respectively. For the purpose of efficiently transferring a signal charge, the minimum values min (L1) and min (L2) of the width L1 and the width L2 satisfy the following Equations (1) and (2): EQU min(L1)=X.sub.etch +2.times.X.sub.overlap-E ( 1) EQU min(L2)=X.sub.etch +2.times.X.sub.overlap-E ( 2)
where X.sub.etch is a minimum width of a polysilicon electrode to be etched, and X.sub.overlap-E is an overlapping width of adjacent polysilicon electrodes required for generating a fringing electrical field.
The first polysilicon electrode 703 and the second polysilicon electrode 704 are formed by a conventional method as is shown in FIGS. 8A to 8C. As is shown in FIG. 8A, the insulating film 702 is formed on the n-type semiconductor substrate 701. Then, the first polysilicon electrode 703 is formed on the insulating film 702. The surface of the first polysilicon electrode 703 is oxidized to form an oxide film 710 thereon. Then, the second polysilicon electrode 704 is formed on the insulating film 702 provided on the n-type semiconductor substrate 701 so as to partially overlap the first polysilicon electrode 703 by X.sub.overlap-D. The surface of the second polysilicon electrode 704 is oxidized to form an oxide film 711 thereon.
Oxide film portions 712 and 713 are formed in a bird's beak shape in the vicinity of a connecting portion of the first polysilicon electrode 703 and the second polysilicon electrode 704. The formation of the oxide film 711 causes an end 714 of the second polysilicon electrode 704 to beturned up. As a result, the overlapping width of the first polysilicon electrode 703 and the second polysilicon electrode 704 is reduced by .DELTA.X.sub.overlap to be X.sub.overlap-E. Thus, the turning up of the electrode should be considered in order to obtain the overlapping width sufficient for effectively generating a fringing electrical field. That is, the minimum value of the width L2 shown in FIG. 7B should satisfy the following Equation (3): EQU min(L2)=X.sub.etch +2.times.(X.sub.overlap-E +.DELTA.X.sub.overlap)(3)
In order to highly integrate a charge transfer device, it is required that .DELTA.X.sub.overlap is nullified and X.sub.overlap-E is made as small as possible.
FIG. 9 partially shows a conventional charge transfer device 800 including a first HCCD 801, a second HCCD 802 and a transfer gate device 803 provided therebetween. The first HCCD 801, the second HCCD 802 and the transfer gate device 803 are positioned in parallel along an X direction. The first HCCD 801 and the second HCCD 802 respectively have first polysilicon electrodes 804 and second polysilicon electrodes 805. The transfer gate device 803 has a third polysilicon electrode 806. In order to transfer a signal charge from the first HCCD 801 to the second HCCD 802 in a Y direction, the first and second polysilicon electrodes 804 and 805 respectively overlap the third polysilicon electrode 806 by Y.sub.overlap, whereby a fringing electrical field is generated. Thus, the charge transfer device 800 needs at least three polysilicon electrodes. The first polysilicon electrode 804 and the second polysilicon electrode 805 have a side in the Y direction longer than that in the X direction. Therefore, the charge transfer in the Y direction is inefficient. The reason for this is that the charge transfer efficiency depends upon the electrode length in the charge transfer direction.